This invention relates to a method of forming a dental coping for use in the preparation of a dental restoration.
A metal coping is used in dentistry in the construction of a dental crown and/or a bridge. The metal coping functions as the under structure of the crown and is usually covered, for reasons of aesthetics, with a fired-on coating of a ceramic porcelain composition or a polymer based veneering material. The metal coping supports the coating and provides the required structural strength and rigidity for the restored tooth to resist the forces of mastication.
A metal coping may be cast from an investment of a wax or plastic pattern of the tooth to be restored. An alternative procedure for forming a precious metal coping which does not require waxing, investing or casting has currently been gaining wide acceptance in the dental profession by both dentists and dental laboratories. This alternative procedure requires the use of a moldable material composition formed from a base material composed of a mixture of high and low fusing temperature metal particles and a binder preferably of dental wax as is disclosed, for example, in U.S. Pat. Nos.: 5,234,343, 5,593,305 and 5,730,600 respectively, each disclosure of which is herein incorporated by reference. The dental material is molded over a die into the shape of the tooth to be restored and heat treated at an elevated temperature above the melting temperature of the low fusing temperature metal particles and below the melting temperature of the high fusing temperature metal particles. Heat treatment transforms the molded structure into a porous metallic shell having the same shape as before heat treatment without suffering any significant shrinkage. The dental wax in the molded material vaporizes during heat treatment leaving the porous metallic shell with a high void volume of preferably above at least 20%. A filler material of metal or ceramic is melted into the porous shell to densify and solidify the shell into a dental coping having the identical shape of the die and in the tooth preparation as prepared by the dentist or dental laboratory. The filler material may be added either in a secondary heat treatment operation or during the primary heat treatment of the dental material.
The base material of high and low fusing temperature metal particles and wax binder may be configured into any geometrical shape for use by the dental laboratory such as, for example, in the form of a thin compacted strip of rectangular geometry. Likewise the filler material which is preferably of a precious metal such as gold or a gold alloy and wax binder may be configured into any geometrical shape preferably corresponding to the shape of the base material.
The method currently employed to form a coping from separate strips of base material and filler material is a labor intensive hand molding procedure in which the base material is cut into pieces each of which is applied by hand to the die. Thereafter the base material is adapted to the die by hand alone or in combination with a hand burnishing tool. An automated mechanism may also be used to adapt the base and filler materials to the die and to mold them over the die. These steps to adapt and mold the material to the die may be accomplished with the help of air pressure, water pressure, mechanical pressure or vacuum. The molded structure of base material is then heat treated to transform the molded structure into a porous metallic shell. Filler material is then melted into the porous shell in a heat treatment operation which may be performed independent of the heat treatment of the base material or alternatively by heat treating both the base and filler materials sequentially in a single heat treatment operation.
The hand molding operation is time consuming and labor intensive. Since the base material is a composition of precious metals and/or alloys the adaptation procedure is carried out in a way to minimize the loss of base material into waste. Moreover, once the base material is placed into contact against the die it may be contaminated and, if so, cannot be readily recycled.
An automated method has been discovered in accordance with the present invention to form a dental coping from a sheet of metallic material and preferably from a first sheet of a base composition of high and low fusing temperature metal particles and a binder and a second sheet of a filler material or a laminate of a base material and filler material with the method resulting in reducing the need for human intervention. The first and second sheets of material may be placed on top of one another to form an single sheet of two layers and/or the base and filler may themselves be represented by multiple layers. The filler material should be of precious metal or ceramic and the base material composition should preferably be relatively soft and malleable and of metal(s) or metal alloys which are compatible for use by the dental profession to restore teeth. The base material and filler material composition taught in the aforementioned patents are the preferred materials.
The automated method of the present invention for forming a dental coping comprises: scanning a three dimensional image of the die of the tooth or teeth to be restored; digitizing the scanned three dimensional image into digital information, storing the digital information in a computer; feeding the digital information from the computer into a computerized numerical control cutting machine; cutting out a section of material of metallic composition into a two dimensional configuration representing a two dimensional lay out of the scanned three dimensional image, adapting the cut out section of material over the die so that the material covers the die surface in close engagement therewith to form a single three dimensional structure having the shape of the die and heat treating the structure into a coping conforming in shape to the die.
In accordance with the present invention when two separate sheets of base and filler material are used a section of each sheet is cut out to form a two dimensional lay out of of the scanned three dimensional image of the die with the cut out section of filler material placed over the molded structure of base material before or after heat treatment. The cut out section of filler material should be equal or different in dimension so that the surface area of the cut out section will fill the porous structure of base material after heat treatment leaving slightly less filler material around the rim which forms the margin.
In a preferred alternative embodiment of the method of the present invention the die of the tooth or teeth to be restored may be formed having at least one reference marker such that the two dimensional lay out of base material will have a complementary reference marker to assist in providing a starting location or for establishing alignment when wrapping the cut out section of base material over the die. In this way the reference marker may be used to facilitate the adaptation of the two dimensional cut out section to the die. An additional reference marker may be formed in the two dimensional cut out section either manually or automatically to provide accuracy during placement and proper alignment in the adaptation of the cut out section of base material to the die.